In my little, old destroyer, there was about 300 in the crew, as I recall. The bridge watch would consist of the OOD, 3 lookouts, a signalman, helmsman, lee helmsman, a quartermaster, possibly a junior OOD, and if there was anything happening, the commanding officer. The navigator and his assistant would be about at dawn and dusk. Then, down below in the combat information center, there would be another 4 or 5 watch standers, plotting other ships from radar and responding to questions from the bridge, e.g., what is CPA on Skunk Bravo? range to to the guide ship (usually the carrier)? etc. Then there were multiple verbal contacts between the bridge and engineering, e.g. request permission to blow boiler tubes (often denied in daylight hours). Speed and course changes to maintain station. One person on sonar, and several dozen in engineering (two firerooms, two enginerooms, etc.). Plus, a lookout on the fantail watching for a man overboard, a gunners mate with a rifle in case a plane crashed and sharks were a possibility, and a man in after steering ready to take over if the main steering failed. Anyway, you get the idea - much different, I'm sure, than a merchant ship.
Looking back on it, the overall manning level was determined by the needs in the event of general quarters, battle stations. In most other times, there were extra hands that needed to be occupied.
